Mobile communication systems employ signal processing techniques against the impact of time variant and frequency selective mobile radio channels to improve the link performance. Equalization is used to minimize intersymbol interference (ISI) caused by multipath fading in frequency selective channels. Since the mobile radio channel is random and time varying, an equalizer needs to identify the time-varying characteristics of the mobile channel adaptively through training and tracking. Time division multiplex access (TDMA) wireless systems such as Global System for Mobile communications (GSM) transmit data in fixed-length timeslots, and a training sequence is included in the timeslot (burst), which is designed to allow the receiver to detect timing information and to obtain channel coefficients through channel estimation for further channel equalization.
GSM is a successful digital cellular technology being deployed worldwide. Currently, GSM networks provide both voice and data service for billions of subscribers and are still expanding. The access scheme of GSM is TDMA. As illustrated in FIG. 1, in the 900 MHz frequency band 100, the downlink 102 and uplink 104 are separated, and each has a 25 MHz bandwidth including 124 channels. Carrier separation is 200 kHz. A TDMA frame 106 consists of 8 timeslots 108 corresponding to one carrier frequency. The duration of a timeslot is 577 μs. For a normal burst, one GSM timeslot includes 114 data bits, 26 training sequence bits, 6 tail bits, 2 stealing bits, and 8.25 guard period bits. Currently, only one user's speech is transmitted in each timeslot.
Eight training sequences for GSM normal bursts are defined in the 3GPP specification (see TS 45.002, “GERAN: Multiplexing and multiple access on the radio path”) and are widely used in practice for burst synchronization and channel estimation in current GSM/EDGE Radio Access Network (GERAN) systems.
With the increase in the number of subscribers and voice traffic, great pressure is added on GSM operators especially within countries with dense population. In addition, efficient use of hardware and spectrum resources is desired as voice service prices drop. One approach to increasing voice capacity is to multiplex more than one user on a single timeslot.
Voice services over Adaptive Multi-user channels on One Slot (VAMOS) (see GP-081949, 3GPP Work Item Description (WID): Voice services over Adaptive Multi-user channels on One Slot) (note: Multi-User Reusing-One-Slot (MUROS) (see GP-072033, “WID”: Multi-User Reusing-One-Slot) is the corresponding study item)) is an ongoing work item in GERAN that seeks to increase voice capacity of the GERAN in the order of a factor of two per BTS transceiver both in the uplink and the downlink by multiplexing at least two users simultaneously on the same physical radio resource, i.e., multiple users share the same carrier frequency and the same timeslot. Orthogonal Sub Channel (OSC) (see GP-070214, GP-071792, “Voice capacity evolution with orthogonal sub channel”), co-TCH (see GP-071738, “Speech capacity enhancements using DARP”) and Adaptive Symbol Constellation (see GP-080114 “Adaptive Symbol Constellation for MUROS (Downlink)”) are three MUROS candidate techniques.
In the uplink of OSC, co-TCH, and Adaptive Symbol Constellation, two users sharing the same timeslot employ GMSK (Gaussian minimum shift keying) modulation with different training sequences. The base station uses signal processing techniques such as diversity and/or interference cancellation to separate two users' data. Similar to the uplink, in the downlink of co-TCH, two different training sequences are used for DARP (Downlink Advanced Receiver Performance) capable mobiles to separate two users. In the downlink of OSC or Adaptive Symbol Constellation, two subchannels are mapped to the I- and Q-subchannels of a QPSK-type or Adaptive QPSK (AQPSK-type) modulation in which the ratio of I-subchannel and Q-subchannel can be adaptively controlled. Two subchannels use different training sequences as well.
FIG. 2 lists eight GSM training sequence codes of 26 bits, each of which has a cyclic sequence structure, i.e., the reference sequence of 16 bits is in the middle and 10 guard bits (5 guard bits are in each side of the reference sequence). The most significant 5 bits and least significant 5 bits of the reference sequence are copied and arranged to append to and precede the reference sequence, respectively. The guard bits can cover the time of intersymbol interference and make the training sequence resistant to time synchronization errors. Each GSM training sequence has ideal periodic autocorrelation properties for non-zero shifts within [−5, 5] when the 16-bit reference sequence is considered only.
In GP-070214, GP-071792, “Voice capacity evolution with orthogonal sub channel”, a new set of eight training sequences of length 26 bits was proposed for OSC, in which each of the new training sequences is optimized in cross-correlation properties with the corresponding legacy GSM training sequence. The new sequences are listed in FIG. 3. It can be observed that these new training sequences do not preserve the cyclic sequence structure as the legacy GSM training sequences.